In an existing vibration sensor, as can be seen in FIG. 17, there are three forces which act on a sphere (movable gravitation element) 70: the force F.sub.1 of gravitational acceleration; the force F.sub.2 of seismic acceleration; and the spring force F.sub.3 of movable member 71. The movement of sphere 70 is determined by the ratio of each of forces F.sub.1, F.sub.2 and F.sub.3 in the direction of an incline.
When a given seismic acceleration G acts on the vibration sensor, the component of force F.sub.2 in the direction of the incline which is due to seismic acceleration G becomes larger than the component of force F.sub.1 in the direction of the incline which is due to the gravitational acceleration which acts on sphere 70. Sphere 70 moves from rest 72 to conical surface 73 and pushes plunger 74 upward. Plunger 74 then actuates projection 75, which in turn pushes movable member 71 upward at pressure point A and causes it to bend. Movable contact 76 on movable member 71 comes in contact with fixed contact 77, which closes the switch and detects the seismic acceleration G.
The prior art vibration sensor described above detects vibration only when the force exceeds a given seismic acceleration G. If we wish to apply different processing according to the magnitude of the seismic acceleration, we have no choice but to employ a number of discrete vibration sensors.